Bioerodible polymers are well known in the art, and the importance of delivery devices manufactured from such polymers has been long recognized. Such devices are valuable because they can contain a beneficial or otherwise active agent that, as the polymer erodes, is delivered at a controlled rate and in an effective amount to the environment of use.
One such family of polymers are the poly(orthoesters) disclosed in U.S. Pat. Nos. 4,070,347, 4,093,709, 4,122,158, 4,131,648, 4,138,344, and 4,155,992, for example. Other polymers are the poly(orthoesters) and poly(orthocarbonates) disclosed in U.S. Pat. No. 4,180,646. A third group of bioerodible polymers are the poly(lactic acids) and the poly(glycolic acids) and mixtures and copolymers thereof. All of these polymers have a controlled rate of erosion to innocuous products when in an aqueous or a biological environment.
Many active agents are released from these bioerodible polymers primarily by dissolution or erosion of the polymer. This is especially true of those agents which, because of high molecular weight or low solubility, for example, do not readily diffuse from a polymer matrix. However, release of beneficial agents by erosion may be undesirable or unaccceptable in many applications because the agent is not released or is released at an insufficient rate for a substantial period of time until erosion of the polymer has begun, and is thus unavailable to the environment for that extended "lag phase" period. Additionally, release through erosion of the polymer is a problem if it is desirable to have a rate of release of the agent which is different from the rate of erosion of the polymer, since release is normally dependent on the erosion rate of the polymer. Also, erosion of the polymer is not always continuous or predictable, so that the reproducibility of a release rate profile is often difficult.
Even where an agent is released from a bioerodible polymer by dissolution in and diffusion from the polymer, such diffusional release is often a slow process and not easily controlled. In general, the use of fillers in the polymeric matrix will slow this process. Additionally, the dissolution rate can be disturbed if the polymer loses its shape, which often happens with certain polymers when they are placed in a particular environment.
Therefore, it would be desirable to provide a means for controlling the delivery of an agent from a bioerodible polymer and to provide a reproducible release rate profile for the agent.